FRESNO — A mosquito that can carry dengue and yellow fever has been found in California, prompting intense eradication efforts in the Central Valley and warnings from officials about how to keep the pest from spreading.

"It could change the way we live in California, if we don't stop it," said Tim Phillips of the Fresno Mosquito and Vector Control District. "Imagine not feeling safe to sit out in your backyard in the afternoons."

The yellow fever mosquito, or Aedes aegypti — a white polka-dotted bug that bites during the day and can lay its eggs in less than a teaspoon of water — was first detected in June in Madera, the Los Angeles Times reported.

"We were shocked," said Leonard Irby, district manager of Madera's abatement program. "We never expected this mosquito in California."

An eradication effort was launched that included agents going door to door to warn people about standing water. But soon the mosquito was found in Clovis and Fowler. It turned up in August in the Bay Area's San Mateo County, and this week in Fresno. Officials are spraying insecticide around infected homes.

"This affects all of California," Irby said. "It requires everyone's help: Turn over plant saucers, wash out dog bowls, remember this mosquito can lay eggs even in the cracks of cement if water is left there for a couple of days." The bug can even lay eggs in flowering plants such as bromeliads.

None of the mosquitoes have been found in the Los Angeles area. Across California, vector control agents constantly trap mosquitoes to look for any invasive species.

Because of a recent infestation of the Asian Tiger mosquito in Southern California, agencies were already monitoring more traps, said Mark Daniel, director of operations for the Greater Los Angeles County Vector Control District.

"There is no evidence of aegypti in Los Angeles. But you can never say with 100% certainty," he said. "Our brother and sister agencies in the Central Valley are being very aggressive and we're on high alert."

So far, none of the trapped mosquitoes have carried disease. The California Department of Public Health reported 200 cases of dengue fever since 2010, all contracted out of the country.

"Presently, dengue, yellow fever and chikungunya do not occur within California," Dr. Gilberto Chavez, deputy director of the California Department of Public Health's Center for Infectious Disease, said in an email. "There is presently no risk of these diseases being acquired locally and traveling within the state."

But the state is warning the medical community to be on the lookout for cases.

Yellow fever is a virus that causes fever, chills, nausea, headache and a prominent backache. Extreme cases cause jaundice, and can be deadly. Dengue causes head and muscle aches, a rash similar to measles and can also kill. Chikungunya is characterized by high fever, rash and months of joint pain.

It is not clear where the bugs are coming from.

The first detections were in affluent neighborhoods. Vector Control agents asked people whether they'd traveled recently or bought plants. Many had. But agents have found no mosquitoes at local nurseries or big box stores and few shared travel destinations, officials said. So far, they've found nothing in common among the infected areas.

As they searched for clues, officials launched a public awareness campaign, including posters at the Big Fresno Fair.

"A lady read the flyer and she remembered earlier thinking, 'Hmm, there are mosquitoes biting during the day. That's different' and noticing bites on her baby," Phillips said. "She went home, trapped a mosquito and called us."

Phillips was on a Department of Public Health conference call about abatement efforts when a biologist identified the mosquito from the woman's house as Aedes aegypti.

He sent a photo of the brown bug with white markings to the other 14 people on the call. There was silence.

"When the aegypti was in Madera there was concern, then Clovis — more concern, then it got to Fowler and it was 'Criminy!'" Phillips said. "But, now Fresno. It's got some people questioning whether we can stop it."

The hope is that winter temperatures will freeze the eggs and eradicate the bugs. But people are often told to bring sensitive plants indoors during a freeze. The mosquito lays eggs just above the water line in containers. After the eggs dry out, they can still survive when more water is added, experts said.

The mosquito has been in some parts of the United States for centuries, arriving on New World ships from Africa. It is present in Florida, Texas and Arizona. But this is believed to be the first time it has reached California.

"We don't want it here," Phillips said. "But it's going to take a heck of a lot to stop it."